undoubtedly considerably warmer than today, and
northwest Europe also lay further south. Chalk forms
a characteristic rock which is exposed in Denmark,
South England and France, and continues under the
southern and middle sections of the North Sea. It is
missing in the north, possibly for palaeoclimatic
reasons, although it is found in northeast Ireland.
Chalk sediments were probably deposited in water
depths of not more than 100–300 m, mostly below
the photic zone. Since chalk is a micritic limestone,
one would not expect it to form a suitable reservoir
rock. The Ekofisk and associated fields are in fact the
world’s only major oilfield in such rocks, and the low
permeability of this fine-grained lithology creates
problems for production.
Pelagic calcareous algae such as coccolithophores
did not become common until the late Jurassic and
early Cretaceous. Consequently we do not have chalk
deposits from older periods. This is an example of
where the rock type is totally dependent on which
organisms were precipitating carbonate. During the
Palaeozoic most carbonate production took place in
shallow water, as there were no planktonic calcareous
algae to form deepwater pelagic carbonates.
5.7.3.7 Lakes and Inland Seas
Carbonate sedimentation in lakes depends on the rate
of weathering and supply of Ca
2+ from older calcareous
sediments and calcium-bearing silicate rocks in
the drainage basin. If there is an ample supply of Ca
2+
and CO
2À
3
also lacustrine sediments may contain
considerable amounts of carbonate, and particularly
at lower latitudes we find pure carbonate deposits. In
cold lakes the solubility of carbonate is high and the
carbonate content in the bottom sediments will mostly
be limited to a few species of bivalves and gastropods.
Temperate lakes often accumulate calcareous muds
called marls, which in some lake sediments contain
dolomite. Algae and certain higher plants often play an
important role in carbonate production in lakes.
The Dead Sea is a good example of an inland sea
where carbonate is precipitated chemically due to
strong evaporation. In Africa and other tropical
areas, lakes will be subject to seasonal evaporation
to dryness, and we may find alternating layers of
biogenic carbonate and chemically precipitated
carbonate. Here, too, algal blooms in the surface
water layer play a major role in precipitating
carbonate. Thicker beds of carbonate below lake floors
can be associated with longer term climate variation.
In Lake Victoria, the sediments older than about
12,000 years have high carbonate content. This is
because the lake nearly dried up during the last glacial
period, when rainfall in the region was lower than
now.
5.7.3.8 Calcareous Tufa Deposits
and Travertine
Tufa is the name of a porous and spongy calcareous
deposit common in limestone areas, usually at the base
of slopes where groundwater emerges at the ground
surface. Accreted in thin layers which often incorporate vegetation, it can build up into deposits several
metres thick. Travertine is a more massive, relatively
dense and sometimes finely banded and laminated
carbonate deposit associated with freshwater springs
or precipitated as speleothems in caves. Both varieties
are precipitated from freshwater supersaturated with
respect to calcium carbonate. Groundwater flowing
through carbonate rock contains CO 2 at a higher partial pressure than at the surface because of the lower
ambient temperature and the higher pressure that is
due to the weight of the overlying water column. The
increased partial pressure of CO 2 promotes increased
dissolution of calcium carbonate from the host
rock. When the water flows out of the rock at the
surface, or reaches a cave, it will regain equilibrium
with atmospheric pressure, causing degassing of CO 2 .
The loss of CO 2 entails that the water may become
supersaturated with regards to calcite, followed
by precipitation of this mineral. This is particularly
effective in summer when the water will quickly warm
up once it emerges. Exposure to light will also cause
biogenic precipitation (photosynthesis) by algae.
5.7.4 Meteoric Water Flow
and Diagenesis
The introduction of meteoric water has a profound
effect on carbonate sediments and their potential as
reservoir rocks.
A proportion of the rainwater (meteoric water) falling on land infiltrates into the groundwater. The flow
of groundwater ultimately is limited by the rate of
recharge by rainwater, which determines the water
table gradient. As long as the water table is above
194
N.-M. Hanken et al.
northwest Europe also lay further south. Chalk forms
a characteristic rock which is exposed in Denmark,
South England and France, and continues under the
southern and middle sections of the North Sea. It is
missing in the north, possibly for palaeoclimatic
reasons, although it is found in northeast Ireland.
Chalk sediments were probably deposited in water
depths of not more than 100–300 m, mostly below
the photic zone. Since chalk is a micritic limestone,
one would not expect it to form a suitable reservoir
rock. The Ekofisk and associated fields are in fact the
world’s only major oilfield in such rocks, and the low
permeability of this fine-grained lithology creates
problems for production.
Pelagic calcareous algae such as coccolithophores
did not become common until the late Jurassic and
early Cretaceous. Consequently we do not have chalk
deposits from older periods. This is an example of
where the rock type is totally dependent on which
organisms were precipitating carbonate. During the
Palaeozoic most carbonate production took place in
shallow water, as there were no planktonic calcareous
algae to form deepwater pelagic carbonates.
5.7.3.7 Lakes and Inland Seas
Carbonate sedimentation in lakes depends on the rate
of weathering and supply of Ca
2+ from older calcareous
sediments and calcium-bearing silicate rocks in
the drainage basin. If there is an ample supply of Ca
2+
and CO
2À
3
also lacustrine sediments may contain
considerable amounts of carbonate, and particularly
at lower latitudes we find pure carbonate deposits. In
cold lakes the solubility of carbonate is high and the
carbonate content in the bottom sediments will mostly
be limited to a few species of bivalves and gastropods.
Temperate lakes often accumulate calcareous muds
called marls, which in some lake sediments contain
dolomite. Algae and certain higher plants often play an
important role in carbonate production in lakes.
The Dead Sea is a good example of an inland sea
where carbonate is precipitated chemically due to
strong evaporation. In Africa and other tropical
areas, lakes will be subject to seasonal evaporation
to dryness, and we may find alternating layers of
biogenic carbonate and chemically precipitated
carbonate. Here, too, algal blooms in the surface
water layer play a major role in precipitating
carbonate. Thicker beds of carbonate below lake floors
can be associated with longer term climate variation.
In Lake Victoria, the sediments older than about
12,000 years have high carbonate content. This is
because the lake nearly dried up during the last glacial
period, when rainfall in the region was lower than
now.
5.7.3.8 Calcareous Tufa Deposits
and Travertine
Tufa is the name of a porous and spongy calcareous
deposit common in limestone areas, usually at the base
of slopes where groundwater emerges at the ground
surface. Accreted in thin layers which often incorporate vegetation, it can build up into deposits several
metres thick. Travertine is a more massive, relatively
dense and sometimes finely banded and laminated
carbonate deposit associated with freshwater springs
or precipitated as speleothems in caves. Both varieties
are precipitated from freshwater supersaturated with
respect to calcium carbonate. Groundwater flowing
through carbonate rock contains CO 2 at a higher partial pressure than at the surface because of the lower
ambient temperature and the higher pressure that is
due to the weight of the overlying water column. The
increased partial pressure of CO 2 promotes increased
dissolution of calcium carbonate from the host
rock. When the water flows out of the rock at the
surface, or reaches a cave, it will regain equilibrium
with atmospheric pressure, causing degassing of CO 2 .
The loss of CO 2 entails that the water may become
supersaturated with regards to calcite, followed
by precipitation of this mineral. This is particularly
effective in summer when the water will quickly warm
up once it emerges. Exposure to light will also cause
biogenic precipitation (photosynthesis) by algae.
5.7.4 Meteoric Water Flow
and Diagenesis
The introduction of meteoric water has a profound
effect on carbonate sediments and their potential as
reservoir rocks.
A proportion of the rainwater (meteoric water) falling on land infiltrates into the groundwater. The flow
of groundwater ultimately is limited by the rate of
recharge by rainwater, which determines the water
table gradient. As long as the water table is above
194
N.-M. Hanken et al.